Generally, a twin-roll continuous casting machine is known as an apparatus applying a Bessemer type continuous casting method, and is used for producing a metal thin sheet by pouring a molten metal between a pair of water-cooled casting rolls and solidifying it.
The production of the thin sheet by a twin-roll continuous casting machine 11 of this kind is carried out as shown in FIG. 3. A molten metal L is poured from above and between a pair of casting rolls 12a and 12b disposed with a predetermined gap between them as shown in the drawing, and these casting rolls cooled inside with water 12a, 12b are rotated inwardly downward. Then, the molten metal L are brought into contact with the casting rolls 12a, 12b and is cooled, and is solidified as the solidified shell S in an arcuate form on the surface of each casting roll 12a, 12b. Each solidified shell S is brought close to the other with the revolution of the casting rolls 12a, 12b and is pressed at the minimum portion of the roll gap (hereinafter referred to as the "roll kiss point") into a casting C having a predetermined thickness. The casting C is pulled down from between the casting rolls 12a, 12b.
In this case, it is the point F (hereinafter referred to as the "solidification start point") at which the molten metal L comes into contact with each casting roll 12a, 12b that solidification of the solidified shell S starts. Each solidified shell S which starts solidifying from the solidification start point F of each casting roll 12a, 12b continues to grow till the roll kiss point, and at this roll kiss point, each solidified shell S is rolled into the casting C having a predetermined thickness.
An associated technology for winding the casting C so produced onto a coiler as-cast and shipping the product is described in Japanese Unexamined Patent Publication (Kokai) No. 58-359.
The gist of the invention disclosed in this prior art resides in the following point. In the method, a runner box encompassed by a frame is defined between a pair of water cooling rolls and a tundish, and the upper surface of a molten steel runner box frame is brought into close contact with the bottom surface of said tundish so that an iron hydrostatic pressure of the molten steel level inside said tundish is allowed to act on the solidified shells formed on the pair of said water cooling rolls.
Because this process can obtain a thin cast strip having a casting thickness equivalent to that of a hot-rolled steel sheet obtained through existing rough rolling and finish rolling, at the time of casting, the process can eliminate the hot rolling step according to the prior art and can drastically reduce the cost of production. However, the steel sheet in the form of the cast strip as it is involves the problem that it is inferior in the aspect of the material.
In other words, according to the invention of the prior art, the casting so produced is used, in the as-cast state, as the product. Therefore, the crystal grain size is great, and both elongation and machinability are low. In other words, satisfactory mechanical strength cannot be obtained. Further, because scales of about 100 .mu.m adhere to the surface of the thin sheet casting as-cast, the surface of the casting is rough and coarse.
Therefore, in order to finish the casting C so casted to the product, there are a method which removes the scales of the casting C after casting, rolls it into a predetermined sheet thickness by hot rolling and winds the resulting strip on a coiler, and another which removes the scales of the casting C after casting, rolls it into a predetermined sheet thickness by cold rolling, anneals the resulting strip and winds it up on the coiler to obtain the product.
A method of finely refining the crystal grain size is described in Japanese Unexamined Patent Publication (Kokai) No. 63-115654.
The gist of the invention disclosed in this prior art resides in the following point. In the method, heat-treatment of cooling a metal thin sheet so casted to a temperature below an A.sub.1 transformation point and then heating it again to a temperature above an A.sub.3 transformation point or heating and holding it at said A.sub.3 transformation point and again cooling it to a temperature below the A.sub.1 transformation point is repeated at least twice in an in-line state.
Japanese Unexamined Patent Publication (Kokai) No. 60-83745 discloses a method of making the texture fine by imparting several times hot-rolling to the casting at a total reduction of at least 20%.
These means all intend to improve the materials by making the metallic texture fine by utilizing recrystallization or transformation. As to factors other than such a metallic structure, the reasons why the materials of the steel sheet in the form of the thin cast strip are inferior have not been sufficiently clarified. When the materials of the thin cast strip are discussed, no prior art references inclusive of the references described above have ever made mention of fluctuation of the materials, that is, variance.
In the invention disclosed in the prior art of JP 63-115654, the crystal grain size is converted to a fine grain size by conducting cooling to the ferrite (.alpha.) zone immediately after solidification and heating to the austenite (.gamma.) zone. However, there remains the problem that the cost of equipment increases because the entire length of the metal thin sheet casting machine used is elongated.
By the way, in order to obtain a product from the casting C by in-line rolling, hot rolling is used more preferably than cold rolling so as to prevent the increase of the entire length of the machine.
Generally, in a case of an inferior material, one is inferior in the characteristics itself and the other is inferior in the fluctuation of the characteristics. In the latter case, though the fluctuation of the material is a predominant problem in a discussion of the steel material since the lowest limit value of the property of the material should be adopted due to product liability as the property of the material, the thin cast strip produced by the process has not sufficiently be studied in view of the point.